Various methods have been used in the past to construct precast walls for retaining earth, soil, sand or other fill, generally referred to as soil. A typical precast wall system is disclosed in U.S. Pat. No. 3,922,864 issued to William Hilfiker on Dec. 2, 1975. The Hilfiker patent illustrates the typical anchored wall using anchor elements which are connected between precast retaining wall panels and deadman members. The deadman members provide resistance to the horizontal forces exerted by the backfill on the wall and function to hold the wall in place. If a solid surface is available, such as rock, the anchor elements can be attached to the rock instead of the deadman members utilizing conventional anchoring techniques.
A disadvantage of such a system is that a considerable amount of labor is required to install the anchor elements and deadman members, and to connect the anchor elements with the proper tension to the retaining wall. Moreover, the anchor elements are typically formed of metal rods (i.e., rebar) which are prone to failure due to occasional defects in the material and progressive weakening due to corrosion of the metal. Furthermore, if the retained soil should settle, vertical motion between the adjacent panels is restricted.
To overcome some of the limitations of utilizing concrete deadman members and metal anchor elements, other retaining wall systems have been developed. As an example, the Reinforced Earth Company (RECO) of Arlington, Va., has developed a system using metal strips which function as both anchor elements for a retaining wall and to provide stability to the soil mass being retained. U.S. Pat. No. 4,961,673 issued Oct. 9, 1990 to Pagnono et al. along with U.S. Pat. Nos. 3,421,326; 3,686,873; 4,045,965 and 4,116,010 to Vidal describe such a wall system. The metal reinforcing strips used in this type of wall system are connected to retaining wall facia panels to hold the retaining wall in position and to provide stability to the backfill material. Such a system uses both the consistency and the density of the backfill material to produce friction between the metal reinforcing strips and the backfill and to render the entire retaining mass stable. The consistency of the backfill is controlled by the size distribution of the particles of the backfill, while the density of the backfill is controlled by compaction of the fill as it is placed in position.
A disadvantage of such a system is that a great deal of expense is involved in obtaining backfill of the proper consistency. If proper materials are available, the size distribution of the particles required can be obtained from local sources using crashing and sifting techniques. Otherwise, backfill must be transported from a location where backfill having the proper consistency is available. In either case, a great deal of expense is involved in obtaining the required backfill at the construction project.
In addition to the costs involved in obtaining and processing the proper backfill, them are significant costs related to the labor intensive process of grading and compacting the backfill. Typically, each layer of reinforcing strips must be precisely compacted to ensure that the requisite friction is provided between the reinforcing strips and backfill for providing a stable soil mass. The labor intensive backfill process thus increases both the costs and installation time associated with the retaining wall.
Additionally, the reinforcing strips used in such a system are subject to the corrosive effect of minerals present in the backfill material. Numerous catastrophic failures have resulted from the effect of unchecked corrosion on the reinforcing strips. Although the metal strips can be galvanized to reduce certain oxidation processes, minerals are present in the backfill material which react with the zinc used in galvanization so as to reduce its effectiveness in a short period of time. Other techniques have also been used to prevent or reduce corrosion of the metal reinforcing strips, including the use of epoxy coatings. Although epoxy coatings are effective against the corrosive chemicals in the backfill, the epoxy coatings are easily scratched during handling, installation, and implementation which exposes the metal strips to the corrosive chemicals. Also, epoxy coatings considerably increase the overall costs of this type of system.
Another problem with this type of wall system is that the system is not flexible and does not accommodate vertical differential settlement. Such setting may result from weak or inconsistent insitu wall foundation material. Since the tensile strength of the reinforcing strip is exceeded when the retained soil mass settles relative to the wall face, failures can occur when the straps or connecting bolts fail. Numerous sliding type connectors have been devised and installed on these wall systems to prevent such failures. Because of the high horizontal loads on the individual connections however, the relative vertical motion desired at the connections has not been achieved. Consequently, high friction forces are generated at these connections as a result of horizontal earth pressure loads on the wall face panels.
Another broad classification of retaining walls systems is the cantilevered wall. A representative cantilevered wall system is disclosed in U.S. Pat. No. 4,050,254 issued Sep. 27, 1977 to Meheen, et al. Design analysis of such cantilevered systems is accomplished by summing the forces of the horizontal tieback element and comparing them with the horizontal component of soil forces acting on the face of the wall. The horizontal tieback base is then increased in size and length until the vertical forces on the tieback element exceed the horizontal component of the soil forces by a predetermined factor of safety. This results in retaining walls having extremely long tieback elements which required a large cut into the backfill material to erect the wall. Consequently, cantilevered walls have not been suitable for implementation as high vertical walls (e.g. over 20 ft) because of the required cut into the backfill. Because of the unsuitability of cantilevered walls as high vertical walls, they have generally been implemented as tiered walls with each successively higher tier set back into retained soil.
U.S. Pat. No. 4,668,129 issued May 26, 1987 to Babcock et al. discloses a multi-tiered retaining wall system which employs soil arching to produce a substantially vertical wall. In this system, each tier acts independently of other tiers and generates shears and soil arching to maintain the stability of the wall. The face of the wall system has a "shiplap" design and each of the column portions of the tieback elements protrudes by a predetermined distance from the face of the retaining wall panels. However, in certain instances, such as when a wall is installed adjacent a roadway, it has been found desirable to have a smooth faced wall without vertical column portions projecting from the wall. A smooth wall reduces the likelihood of contact between the wall and a motor vehicle.
U.S. Pat. No. 4,655,646 issued Apr. 7, 1987 to Babcock et al., discloses a multi-tiered retaining wall system which employs soil arching to produce a vertical wall without vertically aligned protrusions. The multi-tiered retaining wall system uses a plurality of tieback elements which act independently of each other to prevent the transference of forces and moments to lower tiers. This design provides a high degree of flexibility in the horizontal spacing of the tieback elements. Specifically, the horizontal spacing is independent of the width of the retaining wall panel, so as to meet specified technical conditions and allow the use of standard sized prefabricated concrete components. However, the exterior wall surface is not free from protrusions and for retaining wall applications with limited fight of way it has been found desirable to construct the exterior wall surface free of protrusions.
In addition to the shortcomings of the above-cited retaining wall systems, them are additional problems associated with the prior art. Firstly, all of these systems make use of relatively small panels for the wall face (e.g. 25 ft.sup.2). This requires many precast wall panels and reinforcing strips and/or anchor rods to assemble a wall.
In addition, small panel wall systems cannot be built in cut wall applications which require shoring or other top down construction techniques. Conventional methods of top down construction include systems which use shoulder piles or soil nailing. Typically, shoulder piles such as steel H beams, are driven into an existing embankment at the face of the proposed retaining wall and lagging is inserted between the beam flanges. The lagging is displaced downward as the soil below the beams is removed. A permanent face is typically cast over the beams in the field with the wall face reinforcement integrated to the beams. As is apparent, this is an expensive and time consuming process. Accordingly, permanent walls of this type are extremely expensive.
Soil nailing is also a conventional method to accomplish top down wall construction. With soil nailing, rebars are inserted horizontally into an embankment and grouted. The embankment is then removed in lifts as subsequent tiers of rebars are inserted. The exposed vertical soil face of the wall is covered with reinforced shotcrete as the soil is removed. Walls of this type are less costly than the shoulder beam system previously described but are prone to corrosion related failures of the rebar.
In view of these and other shortcomings of the prior art there is a need in the art for improved wall systems in the construction of various structures. Accordingly, it is an object of the present invention to provide an improved precast retaining wall system.
It is another object of the present invention to provide an improved precast retaining wall system that is corrosion resistant and which is formed of components that can be easily and inexpensively protected from corrosion.
It is yet another object of the present invention to provide an improved precast retaining wall system that is self-supporting during assembly and backfilling and which can be assembled without the requirement of erection braces.
It is a further object of the present invention to provide an improved precast retaining wall system that can accommodate settling of the retained soil without excessively loading or tensioning the connecting members or connection points of the system.
It is a still further object of the present invention to provide an improved precast retaining wall system that includes large precast panels that when assembled are free of protrusions and which do not require patching during assembly.
It is a still further object of the present invention to provide an improved precast retaining wall system that includes precast panels that can be leveled and plumbed without external wedges.
It is a still further object of the present invention to provide an improved precast retaining wall system that includes precast panels formed with integral vertical columns having a generally T-shaped cross-section that improves the assembly, support, and load distribution of the panels.
It is another object of the present invention to provide an improved precast retaining wall system that does not require precise backfill grading and which will accommodate different backfill consistencies.
It is a further object of the present invention to provide an improved precast retaining wall system having connecting members and anchors that can be assembled and pretensioned on the backfill side of the wall.
It is yet another object of the present invention to provide an improved precast retaining wall system that incorporates rebar couplers (or threadbar connectors) to facilitate installation of the connecting members of the wall system and which can be preassembled to specification without the necessity of field testing.
It is yet another object of the present invention to provide an improved precast retaining wall system wherein the top wall panels can be extended over the backfill line to provide a wall for pedestrian or vehicular traffic.
It is a further object of the present invention to provide an improved precast retaining wall system adaptable for use as seawall or an erosion control structure and adaptable for use in conjunction with other structures such as sheet pile cut off walls.